The present invention relates to a novel fluorine-containing copolymer, particularly a thermoplastic fluorine-containing copolymer having a fluorine-containing cyclic structure in its main chain. The present invention further relates to a composition for preparing a low reflectance film. The fluorine-containing copolymer can be used as a film-forming component of the composition. A low reflectance film for reducing the light reflectance can be formed on the surface of a glass or transparent substrate by applying the composition thereto.
Japanese Patent Laid-open Publication JP-A-1-131215 discloses a polymer having a fluorine-containing cyclic structure in its main chain. This cyclic structure is a saturated perfluoro ether ring. U.S. Pat. No. 3,418,302 discloses a fluorine-containing polymer obtained by the ring closure polymerization of perfluorodimethylene bis(perfluorovinyl ether). There is known a polymer containing no ether bond, obtained by the cyclization polymerization of a compound represented by the general formula CF2xe2x95x90CF(CF2)XCFxe2x95x90CF2 where X is from 1 to 5, with xcex3-rays (see JP-A-1-131215, page 2, the right column, lines 8-15). U.S. Pat. No. 3,449,304 discloses a homopolymer of hexafluorocyclopentadiene or perfluorodicyclopentadiene, and copolymers of these compounds with hydrocarbon monomers such as fluoroolefins (e.g., tetrafluoroethylene, hexafluoropropene and vinylidene fluoride), methyl methacrylate and acrylonitrile. Published Japanese Translation of PCT International Publication JP-T-11-501685 discloses a copolymer of 49.0 mol % of hexafluoropropene, 50.4 mol % of tetrafluoroethylene and 0.6 mol % of octafluorocyclopentene. J. Poly. Sci., Polym. Chem. Ed. (1977), 15(10), 2411-2425 discloses a graft polymerization on a polymer substrate by a plasma polymerization method. Tetrahedron Lett. (1988), 29(11) 1295-1298 discloses that a conductive material is obtained by an anode electrolysis polymerization.
In cyclization polymerization, the types of monomers to be copolymerized are limited. Thus, it becomes relatively difficult to control the properties of the resulting copolymers. It is disclosed in JP-A-1-131215 that the types of solvents for dissolving a polymer disclosed in this publication are limited. A homopolymer of hexafluorocyclopentadiene or perfluorodicyclopentadiene, or a copolymer of these compounds has a double bond in the ring structure of its main chain.
There is known a method in which the surface of a substrate is covered with a multilayered film having a MgF2 or TiO2 layer and a SiO2 layer, in order to reduce the light reflectance from the substrate surface. The application of this method to large-size articles is limited. In this method, it is difficult to form a practical film on a plastic article, since it is necessary to conduct a baking step when the multilayer film contains MgF2 layer and the like.
There is known another method in which the surface of a substrate is covered with a film of a fluorine-containing polymer such as polytetrafluoroethylene or a copolymer of tetrafluoroethylene and comonomer, in order to reduce the light reflectance. This type of fluorine-containing polymer is inferior in transparency and dissolution in solvent.
There is known the use of amorphous resins such as TEFLON AF (trade name of Du Pont Co. of the USA) and SAITOP (trade name of Asahi Glass Co., Ltd.) for forming a low reflectance film. These resins are high in transparency, but have a limitation of the solvent types for dissolving the resins. Furthermore, these resins can not be used in some cases in which abrasion resistance is required.
It is an object of the present invention to provide a fluorine-containing copolymer having a saturated perfluoro group as a ring structure of its main chain, which can be dissolved in various common solvents and can provide a transparent coated film.
It is another object of the present invention to provide a composition for forming a low reflectance film which is improved in hardness.
It is a further object of the present invention to provide an article comprising a substrate and a low reflectance film covering the substrate.
According to the present invention, there is provided a fluorine-containing copolymer comprising 1-99 mol % of a first repeating unit represented by the chemical formula (1) or (2); and 99-1 mol % of a second repeating unit represented by the general formula -Q- where Q is a bivalent organic group, 
The fluorine-containing copolymer has a number average molecular weight of from 1,000 to 1,000,000 determined in a gel permeation chromatography using polystyrene as a standard material thereof.
According to the present invention, there is provided a process for producing the fluorine-containing copolymer. This process comprises polymerizing 1-99 mol % of an octafluorocyclopentene with 99-1 mol % of a compound having a polymerizing double bond. The above Q is derived from this compound.
According to the present invention, there is provided a composition for forming a low reflectance film. This composition comprises as a film-forming component a fluorine-containing polymer comprising the above first repeating unit. It is optional to use the above fluorine-containing copolymer as a film-forming component of the composition.
According to the present invention, there is provided an article comprising a substrate and a low reflectance film covering the substrate. This film is prepared by a process comprising applying the above composition to the substrate.